We are trying to develop a kit where we need to implement an electrically
isolated serial port between a kit and
a standard PC serial port.

We have a circuit using opto-couplers (4N25's) but it is not reliable. It
seems too
dependant on the IC parameters so that changing the opto-coupler for
another of the same type affects whether the circuit works or not. Is has
to do with the turn-on and turn-off delay times of the opto-coupler.

Does anyone have any suggestions or even a (reliable) circuit to do this?
There is no power supply available on the PC side except what comes out the
serial connector.

>
> We are trying to develop a kit where we need to implement an electrically
> isolated serial port between a kit and
> a standard PC serial port.
>
> We have a circuit using opto-couplers (4N25's) but it is not reliable. It
> seems too
> dependant on the IC parameters so that changing the opto-coupler for
> another of the same type affects whether the circuit works or not. Is has
> to do with the turn-on and turn-off delay times of the opto-coupler.
>
> Does anyone have any suggestions or even a (reliable) circuit to do this?
> There is no power supply available on the PC side except what comes out the
> serial connector.
>
> Any advice gratefully accepted.
>
> regards,
>
> Peter Crowcroft
> DIY Electronics (HK) Ltd
> PO Box 88458, Sham Shui Po, Hong Kong
> Voice: 852-2720 0255 Fax: 852-2725 0610
> Web: http://kitsrus.com Email: .....peterKILLspam@spam@kitsrus.com
> Number One in Electronic Kits Worldwide
> ------------------------------------------------------------------
>
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>We are trying to develop a kit where we need to implement an electrically
>isolated serial port between a kit and
>a standard PC serial port.
>
>We have a circuit using opto-couplers (4N25's) but it is not reliable. It
>seems too
>dependant on the IC parameters so that changing the opto-coupler for
>another of the same type affects whether the circuit works or not. Is has
>to do with the turn-on and turn-off delay times of the opto-coupler.
>
>Does anyone have any suggestions or even a (reliable) circuit to do this?
>There is no power supply available on the PC side except what comes out the
>serial connector.
>
>Any advice gratefully accepted.

It's should just be a matter of designing the circuit for worst case specs.
Optos are notoriously bad for parameter shift over lifetime and also often
have a wide initial parameter spread.
The answers to the following questions would be useful:

>>
We are trying to develop a kit where we need to implement an electrically
isolated serial port between a kit and
a standard PC serial port.
<<

Pricey, but for a one time use, a vendor of ours purchased a "RS-232
opto-isolation module" from a company called Telebyte Technology, Inc.
Approx US $125. Connects to serial port on pc and creates an isolated
serial port with no external power supply.

Since I currently need such a circuit too I'll answer to
Russell's questions if may I take the liberty :)

> It's should just be a matter of designing the circuit for worst case
specs.
> Optos are notoriously bad for parameter shift over lifetime and also
often
> have a wide initial parameter spread.
> The answers to the following questions would be useful:
>
> - How fast a data rate do you wish to be able to handle?

4800bps (9600bps max)

> - What voltage and current is available on the output side?

Current available on the output side is direct drive from PIC RX/TX
pin.
But I also have a DC source of 9..35V/50mA available which can be
stepped down to e.g. 6VDC with a small smps. Cost is not a very big
issue
for this external RS232 isolator. But it's absolutely neccesary that
no power supply is needed on the PC side.

I've already an isolated RS232 application running with a MAX253
oscillator running a small ferrite pushpull transformer to obtain
isolated secondary (PC-) side power for the MAX232. The RX/TX lines
are isolated using 6N136 opto's. While working OK, this solution is
probably the most expensive one and it takes too much board space.
This
last requirement is also important since the isolator has to be housed
in a small external box.

Those optos are SLOW! Investigate with a scope if you need confirmation. The
datasheet performance rating can probably only be reached with heavy bias.

If cost is critical use of the C-B junction of the opto as a reverse biased
photodiode works much better than the conventional circuit, see the Maxim
isolated interface for one way to implement it. Otherwise go with a faster
device.